A power conversion device has the function of converting DC power supplied from a DC power supply into AC power for supply to an AC electrical load such as a rotating electrical machine or the like, or the function of converting AC power generated by a rotating electrical machine into DC power for supply to a DC power source. In order for it to have this type of power conversion function, such a power conversion device includes an inverter circuit incorporating semiconductor devices that have a switching function. The inverter circuit performs power conversion from DC power to AC power, or from AC power to DC power, by these semiconductor devices repeatedly operating to go continuous and to go discontinuous.
Since current is intercepted by this switching operation, spike voltages are generated due to the parasitic inductance present in the circuitry. In order to reduce such spike voltages, along with providing a smoothing capacitor, it is desirable to reduce the inductance of the DC electrical circuit. A technique of keeping down the spike voltages by reducing the inductance is described in Patent Document #1 (Japanese Laid-Open Patent Publication 2002-034268). According to Patent Document #1, the inductance is reduced by shortening the length of the wiring between the smoothing capacitor and the switching elements, so that it is possible to reduce the surge voltage.
A power conversion device that is mounted to a vehicle receives DC power from an in-vehicle power supply, and this DC power is, for example, converted into three phase AC power for supply to a rotating electrical machine for driving the vehicle. The demand upon the rotating electrical machine for driving the vehicle for generation of torque is nowadays becoming large as compared to previous requirements. Due to this, there is also a tendency for the power converted by the power conversion device to become great. Furthermore, a power conversion device that is mounted to a vehicle is used in a high temperature environment as compared with a power conversion device for use with a typical industrial machine. Due to this, with a power conversion device for a vehicle, it is even more desirable to reduce as much as possible the amount of heat generated by the power conversion device itself, as compared to the situation with a typical power conversion device. Since the heat generated by the switching elements included in the inverter circuit constitutes a large proportion of the heat generated by the power conversion device itself, accordingly it is desirable to reduce the amount of heat generated by the switching elements as much as possible.
Since the amount of heat generated increases when the switching elements change over from the non-continuous state to the continuous state, or from the continuous state to the non-continuous state, accordingly it is desirable to reduce the amount of heat generated during the above described changeover operation. In order to reduce this amount of heat generated, a first measure is to shorten the time period for changeover operation of the switching elements. Furthermore, a second countermeasure is to lengthen the interval at which the switching operation of the switching elements is performed, in other words to reduce the overall amount of heat generated by reducing the number of switching operations of the switching elements per unit time. However, with this second countermeasure, there is a limitation on greatly reducing the number of switching operations of the switching elements per unit time, because very large lengthening the interval for switching operation of the switching elements may entail a possibility of reducing the control accuracy.
And, in Patent Document #2 (Japanese Laid-Open Patent Publication 2007-143272), there is disclosed a technique for shortening the time period required for a switching element of an inverter circuit to change over by arranging reduction of the inductance, thus reducing the amount of heat generated per one cycle of switching operation by the switching element.
Patent Document #1: Japanese Laid-Open Patent Publication 2002-034268.
Patent Document #2: Japanese Laid-Open Patent Publication 2007-143272.